U.S. patent application number 11/051373 was filed with the patent office on 2006-08-10 for optical system and method for increasing image resolution and/or dithering in projection applications.
Invention is credited to William M. Bommersbach, Roger S. Carver, Stephen W. Marshall, Steven M. Penn, Frank J. Poradish, Donald A. Powell, Frederick C. Wedemeier, Donald C. Whitney.
Application Number | 20060176323 11/051373 |
Document ID | / |
Family ID | 36779483 |
Filed Date | 2006-08-10 |
United States Patent
Application |
20060176323 |
Kind Code |
A1 |
Bommersbach; William M. ; et
al. |
August 10, 2006 |
Optical system and method for increasing image resolution and/or
dithering in projection applications
Abstract
An optical system for projecting an image having x and y axes
onto a image plane is provided. The system includes an SLM device
spaced from the image plane, the SLM device having a plurality of
pixels operable to project pixels of the image onto the image plane
and positioned such that the individual pixels of the projected
image are oriented at substantially 45 degrees relative to the x
and y axes of the image. The system further includes an optic
element disposed between the SLM device and the image plane and a
linear displacement device operatively connected to and operable to
selectively displace at least one of the SLM device and the optic
element. A method for projecting an image onto a image plane is
also provided.
Inventors: |
Bommersbach; William M.;
(Richardson, TX) ; Whitney; Donald C.; (Allen,
TX) ; Wedemeier; Frederick C.; (Richardson, TX)
; Carver; Roger S.; (Richardson, TX) ; Penn;
Steven M.; (Plano, TX) ; Marshall; Stephen W.;
(Richardson, TX) ; Poradish; Frank J.; (Plano,
TX) ; Powell; Donald A.; (Dallas, TX) |
Correspondence
Address: |
TEXAS INSTRUMENTS INCORPORATED
P O BOX 655474, M/S 3999
DALLAS
TX
75265
US
|
Family ID: |
36779483 |
Appl. No.: |
11/051373 |
Filed: |
February 4, 2005 |
Current U.S.
Class: |
345/697 ;
359/855 |
Current CPC
Class: |
G03B 21/005 20130101;
H04N 9/3114 20130101 |
Class at
Publication: |
345/697 ;
359/855 |
International
Class: |
G09G 5/02 20060101
G09G005/02 |
Claims
1. An optical system for projecting an image having x and y axes
onto a image plane, the system comprising: an SLM device spaced
from the image plane, the SLM device having a plurality of pixels
operable to project pixels of the image onto the image plane and
positioned such that the individual pixels of the projected image
are oriented at substantially 45 degrees relative to the x and y
axes of the image; and a modulating device operable to create
cyclical relative movement between the projected image and the
image plane.
2. The system of claim 1 wherein the relative movement is in at
least two dimensions.
3. The system of claim 2 wherein the relative movement in at least
one dimension is approximately equal to one half the diagonal
length of at least one of the projected pixels.
4. The system of claim 1 further comprising an optical element
disposed between the SLM device and the image plane.
5. The system of claim 4 wherein the optical element is a double
convex lens or a plane-parallel plate.
6. The system of claim 4 wherein the modulating device is a linear
displacement device connected to and operable to selectively
displace at least one of the SLM device and the optical
element.
7. The system of claim 6 wherein the linear displacement device is
selected from the group consisting of a motor, voice coils, and
poled piezoelectric elements.
8. The system of claim 1 wherein the modulating device comprises an
acousto-optic or an electro-optic modulator disposed between the
SLM device and the image plane.
9. A method for projecting an image having x and y axes onto a
image plane, the method comprising: providing an SLM device spaced
from the image plane, the SLM device having a plurality of pixels
operable to project pixels of the image onto the image plane;
positioning the SLM device to orient the individual pixels of the
projected image at substantially 45 degrees relative to the x and y
axes of the image; and creating relative movement between the
projected image and the image plane.
10. The method of claim 9 wherein creating relative movement
comprises displacing the SLM device a length approximately equal to
one half the diagonal length of at least one of the projected
pixels.
11. The method of claim 9 further comprising disposing a modulating
device between the SLM device and the image plane.
12. The method of claim 11 wherein creating relative movement
comprises modulating the modulating device to affect the path of
the projected image.
13. An optical system for projecting an image onto a image plane,
the system comprising: an SLM device spaced from the image plane,
the SLM device having a plurality of pixels operable to project
pixels of the image; and a mirror spaced from the SLM device, the
mirror being movable about an axis defined through the center of
the mirror and being operable to redirect the pixels projected from
the SLM device to the image plane.
14. The system of claim 13 further comprising a display lens
disposed between the mirror and the image plane.
15. The system of claim 13 wherein the mirror is positioned on a
motor, the motor being operable to selectively rotate the
mirror.
16. The system of claim 13 wherein the mirror is connected to a
pair of linear actuators, the linear actuators being operable to
selectively displace the mirror.
17. The system of claim 16 wherein the linear actuators are
selected from the group consisting of voice coils and poled
piezoelectric elements.
18. The system of claim 13 wherein the SLM device is a DMD device.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is related to U.S. patent application Ser.
No. (Docket Number TI-35915) entitled "Optical System and Method
for Increasing Image Resolution and/or Dithering in Printing
Applications."
TECHNICAL FIELD
[0002] Disclosed embodiments herein relate generally to the field
of image display systems using Spatial Light Modulators (SLMs), and
more particularly to methods for improving the visual quality of an
image.
BACKGROUND
[0003] SLMs are used in many applications, two of which are display
systems and printing systems. SLMs are increasingly being used
because they have the benefit of high resolution while consuming
lower power and less bulk than conventional Cathode Ray Tube
technology. Examples of two well known SLM devices include Liquid
Crystal Displays (LCDs) and Digital Micromirror Devices (DMDs). The
SLM will typically consist of an array of picture elements, or
pixels, which modulates light according to a spatial pattern on the
device.
[0004] Generally, an LCD system used in projection systems consists
of three LCD panels in which the primary colors, red, green, and
blue, are combined from the three panels to form the desired image.
An LCD panel consists of a liquid crystal material sandwiched
between transmissive layers and divided into an array of individual
picture elements. Each picture element can individually have an
electric signal applied that causes the material to align in
predictable ways. Light is passed to the panels and individual
picture elements either allow the light to pass through or block
the light. By modulating the opening and closing of the picture
elements, an image is produced which is then directly viewed or
projected on to the image plane or display plane through a display
lens. Traditionally, the array is aligned with the horizontal and
vertical axes.
[0005] Another version of an LCD system is known as Liquid Crystal
on Silicon, which combines some of the features of an LCD panel
with a DMD device. An LCD type device is placed over a reflective
surface and the individual picture element either allows light to
pass to the reflective surface or light is blocked from the
reflective surface. The reflected light corresponding to the
picture element is passed through the imaging system, usually
combining three separate color images of red, green, and blue to
the display plane.
[0006] DMDs are micromechanical devices that typically include an
array of small reflective surfaces, or mirrors, on a semiconductor
wafer to which an electrical signal is applied to deflect the
mirror and change the reflected light applied to the device. A
DMD-based system is created by projecting a beam of light to the
device and selectively altering individual micro-mirrors with image
data, and directly viewing or projecting the selected reflected
portions to an image plane such as a display screen or printing
surface. Each individual micromirror is addressable by an
electronic signal and makes up one "display element" of the DMD. In
many contexts, a single pixel of a DMD or other SLM device
corresponds with a single image pixel in a projection display or
printing context, but there are other ways to address the SLM
device in order to comprise a pixel element of more than or less
than a single SLM device pixel.
[0007] Further, techniques are sometimes desired for dithering of
the displayed or printed images are provided in the art so that
jagged lines, screen-door effects, temporal anomalies, and other
discontinuities and undesirable image effects are reduced.
SUMMARY OF THE SYSTEM AND METHOD
[0008] Disclosed herein are systems and methods for improving an
image from a spatial light modulator, whose elements are viewed,
projected or printed, where such elements are positioned such that
their projection onto an image plane is approximately at a 45
degree rotation from the x and y axes of the image plane. The
disclosed systems and methods further provide for the relative
motion of the projected pixels in the image plane (whether for
image display or printing) in order to increase image resolution
and/or effect image dithering.
[0009] In the DMD context, the micro-mirrors of the devices are
often referred to as the "pixels" of the DMD, as distinguished from
the pixels of an image. The pixels of an image may individually
consist of an image element formed by one or more pixels of the
DMD, or in contrast, multiple image elements may be formed by the
spatial displacement of image projection from a single DMD
pixel.
[0010] Generally, projecting an image from an array of SLM pixels
is accomplished by loading memory cells connected to the pixels.
Once each memory cell is loaded, the corresponding display elements
are reset so that the corresponding display element is turned "ON"
or "OFF" in accordance with the ON or OFF state of the data in the
memory cell. For example, to produce a bright spot in the projected
image, the state of the pixel may be ON, such that the light from
that pixel is directed out of the SLM and into a projection lens.
Conversely, to produce a dark spot in the projected image, the
state of the pixel may be OFF, such that the light is directed away
from the projection lens.
[0011] Modulating the beam of light with a micromirror is used to
vary the intensity of the reflected light. Although the
micro-mirrors can be moved relative to the bias voltage applied,
the typical operation is a digital bistable mode in which the
mirrors are fully deflected at any one time. Generating short
pulses and varying the duration of the pulse to an image bit
changes the time in which the portion of the image bit is reflected
to the image plane versus the time the image bit is reflected away,
therefore distributing the correct amount of light to the image
plane. This technique described above is Pulse-Width Modulation
(PWM) and is used to achieve the level of illumination in both
black/white as well as color systems.
[0012] For generating color images with a DMD system, one approach
is to use three DMDs, one for each primary color of red, green, and
blue (RGB). The light from corresponding pixels of each DMD is
converged so that the viewer perceives the desired color. Another
approach is to use a single DMD and a color wheel having sections
of primary colors. Data for different colors is sequenced and
synchronized to the color wheel so that the eye integrates
sequential images into a continuous color image. Another approach
uses two DMDs, with one switching between two colors and the other
displaying a third color.
[0013] A common artifact of using individual picture elements to
produce an image is that the resulting images may show the gaps
that exist between picture elements. LCD display systems usually
have a larger perceived gap than a comparable DMD based system when
using a similar density of picture elements. The grid produced is
also known as the screen-door effect and will become more
pronounced as the image size is increased. By using a greater
number of picture elements to produce the image, the effect can be
reduced with the trade-off in higher cost and system complexity.
Other discontinuities and structures on the SLMs can also distract
from the desired image uniformity. A method is desired that reduces
the screen-door effect and other artifacts while maintaining image
clarity.
[0014] Printing applications for SLMs include photographic and
electrophotographic printing, both of which are used to print
pictures, characters, and drawings. While the traditional
photographic printing using conventional film and optics is still
in wide use, digital imaging has become popular and continues to
grow. Digital images may be created directly with digital cameras,
may be computer generated, or may be scanned from conventional
photographs or film. Printing to photosensitive materials has many
applications, some of which include printing directly to
photographic paper, creating a master negative, and producing a no
loss film master. The early method of reproducing digital images to
photosensitive material from a CRT was expensive and had
shortcomings such as insufficient phosphor response for certain
colors when operating at high print speeds and poor resolution.
SLMs offer advantages in the area of photographic printing such as
high-speed imaging and lower cost. When used for photographic
printing, the SLM does not need to operate at high frame rates and
an SLM system may be designed with a sequential color system using
one SLM module versus an additive system consisting of three SLMs,
one for each color. Laser systems have also been used in
photographic printing. However, laser systems use rotating mirrors
that make them bulky, complex and expensive. Traditional photo
paper does not work with a laser system and special paper adds
additional cost.
[0015] Electrophotographic printing using an SLM is similar to a
laser printer in that light is used to create an image on the
printer drum. In the LCD context, a halogen or other light source
is shone through an LCD panel, and the panel either lets the light
pass or blocks the light, thereby creating image pixels on the
drum. An LCD printer is sometimes referred to as a crystal shutter
printer. Although not as well known as laser printers, LCD printers
can produce print quality equivalent to that of laser printers. DMD
printers work in a similar fashion to LCD printers. A light source
is reflected from a DMD device and the resulting image is projected
onto a charged print drum. Depending on the drum type, light
photons either charge or discharge the drum where they strike and
toner material is attracted to the charged or discharged areas
respectively. The imaging material, which is also charged, passes
over the drum and attracts the toner material onto the imaging
material where the toner is typically fused to the print material
by heat.
[0016] In both display systems and printing systems, increasing the
resolution is a desired benefit that is directly perceived by the
user. When utilizing an SLM device in an orthogonal array for
either display or printing, the number of elements on the SLM
device typically limits the resolution. Resolution is especially
important in digital printing, where print densities of at least
300 pixels per inch are desired. An 8 inch by 10 inch image would
require 2400 by 3000 pixels respectively. For an image projection
or film created by an SLM, higher resolutions are desired because
the resulting image is projected onto larger display planes.
[0017] Disclosed in this application is the projection of image
pixels that are oriented with their axes at 45 degrees from the x
and y axes of the projected images that they collectively form,
resulting in their forming the collective image with diagonal or
diamond-shaped pixel images. This 45-degree rotation of the SLM
array has the advantage of increasing the image resolution while
using a smaller dynamic image offset relative to a displacement
approach when the axes of the pixels are aligned with that of the
collective image. For example, the image resolution can be enhanced
by a factor of two while using a dynamic image offset of only 1/2
the diagonal size of a projected pixel, whereas the same increase
in image resolution in an aligned system would require displacement
of a full projected pixel length. Advantages of rotating the image
array include but are not limited to reducing the visible gaps
between elements and reducing other discontinuities. It is
additionally possible to move the projected image on the display
plane or print medium in order to increase image resolution and/or
to perform image dithering.
[0018] In one embodiment, the method comprises the orientation of
the SLM in the optical path such that the projection of the SLM
pixels is oriented at a 45 degree angle and wherein the image data
from the SLM is presented to a print material that is stepped in
increments of less than the diagonal length of a projected pixel,
to effectively enhance the resolution of the SLM array. For
example, the print material can be stepped in increments of 1/2 of
the projected diagonal pixel size to effectively double the
resolution of the SLM array. Advantages of this method include
increasing the resolution and having the ability to print at high
speeds through a cumulative exposure method.
[0019] Another method is provided that reduces the exposure
complexity of the first embodiment. Complexity of varying pulse
widths is replaced by adding an additional bit to select between
short and long pulses.
[0020] Printing gray-scale images is accomplished in yet another
embodiment that comprises an SLM array rotated 45 degrees from the
typical orthogonal or aligned position where the image from the SLM
is projected onto an Organic PhotoConductor drum internal to an
electrophotographic printer. By rotating the SLM array and moving
the drum in increments of less than the length of a projected
pixel, the resolution is enhanced. For example, the print material
associated with the drum may be moved a length corresponding to 1/2
the diagonal length of a projected pixel to double the image
resolution. Degrees of gray are also enhanced by the cumulative
exposure that can be utilized since multiple rows are projected
onto the drum simultaneously.
[0021] Yet another embodiment is presented in which an SLM array is
rotated 45 degrees from the typical orthogonal position and the SLM
array is displaced by the distance of 1/2 of a projected diagonal
pixel to increase the resulting image resolution on the display
screen. By properly synchronizing the image planes with the dynamic
displacement of the SLM, the changing image location on the display
screen results in additional addressable picture element locations.
Enhanced resolution is the primary advantage, which reduces the
visible artifacts.
[0022] In other embodiments, linear actuators such as voice coils
or piezo electric devices are used to dynamically displace a mirror
assembly in the optical path. The actuators allow for image
displacement along a single axis or multiple axes and can increase
the picture element light coverage in the gap area between picture
elements and can smooth coverage in other discontinuances. This
embodiment has the further advantage of providing exceptional
control over the amount of image displacement, and can be used not
only to increase image resolution by using a single SLM pixel to
expose at least two pixels in the projected image, but can also be
used for dithering in order to reduce artifacts.
[0023] Additional embodiments include displacing one or more
optical elements in the projection system to displace the projected
image. The displacement of the optical elements may be accomplished
via any number of techniques. Optical elements include but are not
limited to mirrors, lenses, and plane-parallel plates. It may also
be possible to displace the projected image through non-mechanical
means, such as by reshaping an optical element or by varying its
refractive index. The resulting movement of the projection may be
lateral, circular, or elliptical, or it may be more complex,
according to the pixel displacement and/or dithering needs.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] Reference is now made to the following detailed description
of the described embodiments, taken in conjunction with the
accompanying drawings. It is emphasized that various features may
not be drawn to scale. In fact, the dimensions of various features
may be arbitrarily increased or reduced for clarity of discussion.
In addition, it is emphasized that some components may not be
illustrated for clarity of discussion. Reference is now made to the
following descriptions taken in conjunction with the accompanying
drawings, in which:
[0025] FIG. 1 illustrates one embodiment of a digital printing
system for use with print material, which uses an SLM having a DMD
therein to generate image data from an input signal;
[0026] FIG. 2 illustrates a color wheel consisting of three primary
colors for use in a digital printing system;
[0027] FIG. 3 illustrates a portion of print material including
pixel locations aligned for progressive exposure from a portion of
an SLM array rotated 45 degrees from a typical orthogonal
pattern.;
[0028] FIGS. 4a-k illustrates the progressive alignment and
cumulative exposure of the portion of print material of FIG. 3 as
rendered by the SLM array;
[0029] FIG. 5 illustrates the exposure portion of a
electrophotographic printing system using an SLM array rotated 45
degrees from a typical orthogonal pattern and;
[0030] FIG. 6 illustrates a display system using an SLM array
rotated 45 degrees from a typical orthogonal pattern where the SLM
array may be dynamically repositioned;
[0031] FIG. 7 illustrates a pair of micro-mirrors of a DMD
device;
[0032] FIG. 8 illustrates the two micro-mirrors of FIG. 7 in a
tilted position;
[0033] FIG. 9 illustrates a tilted mirror mounted to a motor
shaft;
[0034] FIG. 10 illustrates an optical path utilizing a tilted
mirror mounted to a motor shaft;
[0035] FIG. 11 illustrates a conic projection resulting from a
tilted display plane;
[0036] FIG. 12 illustrates a picture element movement in a dynamic
optical path;
[0037] FIG. 13 illustrates a picture element movement in a manner
that increases the perceived resolution of a display system;
[0038] FIG. 14 illustrates a cross-sectional view of a voice
coil;
[0039] FIG. 15 illustrates a mirror attached to a rotating point
and additionally attached by two voice coils;
[0040] FIG. 16 illustrates a poled piezoelectric ceramic
element;
[0041] FIG. 17 illustrates a dynamic optical path using a moveable
lens system; and
[0042] FIG. 18 illustrates a dynamic optical path using a moveable
plane-parallel plate.
DETAILED DESCRIPTION
[0043] As discussed previously, an SLM device comprises an array of
fixed picture elements that form an image, which may be projected
onto print material or projected onto a display plane. While SLM
technologies differ in the methods in which light is presented to
form the display image, they all have separate picture elements,
which are normally arranged in an orthogonal pattern. Increasing
the size of a projected image causes artifacts to become larger and
more pronounced which can result in a distraction to the viewer. As
display technology has progressed, higher resolution imaging
devices have reduced the effect of artifacts with the trade-off in
higher system cost. Although the higher resolution display systems
may reduce the artifact effects when compared to a similar sized
projected image, a higher resolution display will show the
artifacts as the projected image is increased in size. Images that
are printed need high resolution because artifacts are easily
perceived on a static image.
[0044] The proposed systems and methods in this description will
use DMD-based systems as exemplary embodiments, but the the systems
and methods described can be applied to other types of display
systems using individual picture elements.
[0045] FIG. 1 illustrates a printing system 100 that uses a DMD
device 104 to project an image onto a photosensitive or print
material. A light source 102 is projected through a color wheel 103
by the use of lens 105. Lens 106 collimates the light from the
color wheel and applies the light onto DMD 104, where the image is
formed by electrical signals applied to the selective picture
elements. A projection lens system 107 then projects the image onto
the print material surface 108. An increase in the resolution is
accomplished by orienting the DMD device such that the projected
pixels of the resulting image are oriented at 45 degrees relative
to the x and y axes of the print material 108. The individual
pixels are accordingly presented to the print material as a diamond
shape (see, e.g., FIG. 3). By moving the print material by a
distance of less than a length of one of the projected pixels, in
either the horizontal (x) or vertical (y) direction followed by
exposing the material in at least two exposure phases, the image
resolution can effectively be increased. In some embodiments, the
print material may be moved a distance corresponding to 1/2 the
diagonal length of at least one of the projected pixels, to
effectively double the image resolution. The print material can be
linearly moved in either of the horizontal (x) or vertical (y)
direction in a variety of manners, including via a linear actuator.
The actuator can be used to translate the image on the print
material in a direction transverse to the progression of the print
material in the printer. Additionally, the array can be scanned
onto the print surface as it progresses through the printer to
provide an infinite number of possible effective pixels in the
scanning direction.
[0046] FIG. 2 illustrates 6 exposure regions on a color wheel 103,
which corresponds to exposure phases. For this embodiment, the
color wheel 103 is utilized to provide primary colors to the print
material for the production of color images. The first red region
210 is denoted by R.sub.0 and corresponds to the first phase in the
exposure sequence, phase 0. R.sub.0 is followed by regions 211,
212, 213, 214 and 215 corresponding to R.sub.1, G.sub.2, G.sub.3,
B.sub.4, and B.sub.5 respectively. FIG. 3 illustrates a section 303
of a DMD array 104 that has been rotated such that its individual
pixels project to be oriented at a 45-degree rotation relative to
the x and y axes of the print material orientation as shown. The
illustrated DMD section 303 includes 18 mirrors that are capable of
exposing twice as many pixels on the print material 301 when the
material is stepped in increments of, for example, 1/2 the
projected pixel. A pixel location 302 on the print material 301 is
shown for reference.
[0047] The input image is sampled at every other picture element in
phase 0, exposing the corresponding area (pixels) on the print
material. FIG. 4a illustrates the first exposure sequence as the
pixel 302 and every other pixel afterwards on the row is in a
position to be exposed. The appropriate mirrors on the DMD are
moved to an ON and then to an OFF position to create a light pulse
at the time the color wheel is located at the position for phase 0
corresponding to red, R0. An exposure reference, shown to the
right, illustrates the cumulative exposure to the print material.
The first cell 402 in the exposure reference is representative of
the cumulative exposure to the first pixel location 302. FIG. 4b
illustrates the material advanced to the next location where the
proper picture elements are activated to expose the phase 1 areas.
For the exposure in phase 1, the color wheel is progressed to the
R.sub.1 section of the wheel and the mirrors are again pulsed. FIG.
4c illustrates the third exposure sequence where green, G.sub.2, is
the active position of the color wheel. As the mirrors are pulsed,
pixel 302 accumulates the green exposure and the cumulative colors
are shown in the exposure reference cell 402. FIG. 4d illustrates
phase 3 corresponding to G.sub.3 and FIG. 4e and FIG. 4f illustrate
the blue exposure sequences, phases 4 and 5 respectively. The
reference pixel 302 has been exposed to the three primary colors as
shown in the exposure reference cell 402. In the next exposure
sequence, the row that contains pixel 302 moves from the active
exposure area. The color wheel has also made a complete revolution
and is ready to restart phase 0 corresponding to the color R.sub.0.
Exposure sequence 7 is illustrated in FIG. 4g where the appropriate
mirrors are once again pulsed to expose red to the proper pixels
that are aligned with the array. Exposure sequences 8, 9, 10 and 11
are illustrated in FIG. 4h, FIG. 4i, FIG. 4j, and FIG. 4k
respectively. The final and 12.sup.th sequence moves the print
material from the exposure section of the SLM array 303. In this
simplified approach, any number of methods may be incorporated to
provide a proper exposure level. Such methods may include Pulse
Width Modulation (PWM). Additionally, the cumulative exposure
method lends itself to high-speed printing, especially on
continuous media such as film.
[0048] In another embodiment, a system of the first embodiment is
used with an exposure algorithm not using PWM to generate varying
shades of gray or colors. As described below, desired color images
can be produced by using the same three primary colors and 12
exposure phases of the first embodiment.
[0049] Table 1 below shows a two-bit exposure algorithm example for
providing exposure data. Two bits correspond to pixel values and
the exposure phases 0 though 12 are represented with four phases in
each of three colors. Color wheel 103 would be further divided to
have 12 regions consisting of 4 regions for each color.
TABLE-US-00001 Exposure Phases Red Green Blue Pulse Type Short Long
Short Long Short Long Pixel Values 0 1 2 3 4 5 6 7 8 9 10 11 0 0 0
0 0 0 0 0 0 0 0 0 0 1 0 0 1 1 0 0 1 1 0 0 1 1 2 1 1 0 0 1 1 0 0 1 1
0 0 3 1 1 1 1 1 1 1 1 1 1 1 1
[0050] To control the exposure level in this embodiment, two pulse
durations are provided to expose the print material. Phases 0, 1,
4, 5, 8 and 9 are short pulses, while phases 2, 3, 6, 7, 10 and 11
are long pulses that create additional exposure intensity. An
exposure sequence similar to first embodiment may be used where the
print material is moved and synchronized with the position of the
color wheel. In some contexts, this second embodiment may be less
complex than the first.
[0051] FIG. 5 illustrates another embodiment in which the
diamond-shaped pixels are projected onto an Organic PhotoConductor
(OPC) drum of an electrophotographic printer or copier. The figure
thus generally illustrates an electrophotographic system 500 using
a DMD array 502 that is oriented such that its projected pixels are
rotated 45 degrees relative to the x and y axes of a resulting
image. For the purpose of providing a simple example, only a
portion of the DMD array consisting of 3 rows by 11 columns is
illustrated. A typical DMD array 502 might have 1000 or more
mirrors per row and although the illustration only shows one row
projected onto the OPC drum, multiple rows may be activated and
projected to the drum at the same time. As the OPC drum 503
rotates, image data is transferred from the exposure data memory
507 to the DMD array 502 in accordance with the desired "ON" or
"OFF" state of the selective mirror elements. The drum 503 may be
rotated in a variety of manners, including via a rotational
actuator (not shown). A light source 506 is reflected from the
selective mirror elements and projected by optics 504 onto the
photosensitive area 505 of the OPC drum 503. By rotating the OPC
drum 503 a distance of less than a diagonal length of at least one
of the projected pixels, the horizontal resolution can be enhanced
and the resolution in the drum rotation direction is also enhanced.
In some embodiments, the OPC drum may be moved a distance
corresponding to 1/2 the diagonal length of at least one of the
projected pixels to effectively double the image resolution. The
resolution in the drum rotation direction is only limited by the
resolution of the steps and exposure time desired. The gray scale
may be improved by using accumulative exposure onto the drum. The
cumulative exposure as described in U.S. Pat. No. 5,721,622,
entitled "Grayscale Printing with Spatial Light Modulator and
Sliding Window Memory," which is hereby incorporated by reference
herein. Other exposure techniques such as PWM may be used in
accordance with data delivery to obtain exposure levels. U.S. Pat.
No. 5,461,411, entitled "Process and Architecture for Digital
Micromirror Printer," which is hereby incorporated by reference
herein, describes additional methods for generating gray
scales.
[0052] Cumulative exposure sequences in the embodiment presented
here correspond to shades of gray and each exposure would decrease
the charge in a system where the OPC drum is positively charged.
Once the drum is charged, the print material, which is also
charged, passes over the drum and attracts the toner material onto
the imaging material. As the print material leaves the drum, the
toner is typically fused to the print material by heat. An SLM
printer using this method has the advantage of higher resolution as
a result of the effective doubling (for example) of exposure areas.
Having multiple rows available in conjunction with the cumulative
exposure method, the printer would be capable of operating at high
speeds because an entire row is exposed at a single time in
comparison to scanning across a row.
[0053] FIG. 6 illustrates an embodiment using a DMD array 604 whose
projection is tilted at 45 degrees relative to the x and y axes of
the display plane 608. The illustrated projection display system
600, using DMD 604 therein, may be used to generate moving or
static images. A linear displacement device 602 may also be used to
displace the DMD device or another optical element in the optical
path between the DMD device 604 and the display plane 608 such that
the projected image relative to the display plane 608 is displaced
by a distance of less than a diagonal length of one of the pixels
projected onto the display plane for the purpose of providing
additional addressable pixel locations on the display plane 608. In
some embodiments, the displacement may correspond to 1/2 of the
diagonal length of a projected pixel. For the purpose of providing
a simple example, only the functions significant to increasing the
resolution are shown in FIG. 6.
[0054] A comprehensive description of a DMD-based digital display
system is set out in U.S. Pat. No. 5,079,544, entitled "Standard
Independent Digitized Video System," and in U.S. Pat. No.
5,526,051, entitled "Digital Television System," and in U.S. Pat.
No. 5,452,024, entitled "DMD Display System." Each of these patents
is assigned to Texas Instruments Inc. and each is incorporated by
reference herein.
[0055] The input image signal feed into the signal interface 603
may be from a television tuner, MPEG decoder, video disc player,
video cassette player, PC graphics card, or the like. In fact, an
analog signal may also be the initial image signal, in which case
the signal interface 603 would also contain an analog-to-digital
converter to convert the incoming image signal to a digital data
signal. Processing system 605 prepares the data for display by
performing various pixel data processing tasks. Processing system
605 may include whatever processing components and memory are
useful for various corrections and conversion. Once the processing
system 605 is finished with the data, a display memory module 606
receives processed pixel data from the processing system 605. The
display memory module 606 formats the data, on input or on output,
into bit-plane format, and delivers the bit-planes to the DMD 604.
It is understood that the signal interface 603, the processing
system 605, and the display memory module 606 may be collectively
referred to as the image input apparatus. Of course, the image
input apparatus is not limited to the aforementioned devices and
systems, but may be any device and/or system that operates to
provide image data to the DMD 604. The bit-plane format permits
single or multiple pixels on the DMD 604 to be turned ON or OFF in
response to the value of one bit of data, in order to generate one
layer of the final display image. Although not shown, a sequence
controller associated with the display memory module 606 and the
DMD 604 may be used for providing reset control signals to the DMD
604, as well as load control signals to the display memory module
606.
[0056] Although this description is in terms of an SLM having a DMD
604 (as illustrated), other types of SLMs could be substituted into
display system 600. Details of a suitable SLM are set out in
commonly owned U.S. Pat. No. 4,956,619, entitled "Spatial Light
Modulator," which is hereby incorporated herein by reference
herein. In the case of the illustrated DMD-type SLM, each piece of
the final image is generated by one or more pixels of the DMD 604,
as described above. The SLM uses the data from the display memory
module 606 to address each pixel on the DMD 604. The "ON" or "OFF"
state of each pixel forms a black or white piece of the final
image, and an array of pixels on the DMD 604 is used to generate an
entire image frame. Each pixel displays data from each bit-plane
for a duration proportional to each bit's PWM weighting, which is
proportional to the length of time each pixel is ON, and thus its
intensity in displaying the image. In the illustrated embodiment,
each pixel of DMD 604 has an associated memory cell to store its
instruction bit from a particular bit-plane.
[0057] For each frame of the image to be displayed in color, Red,
Green, Blue (RGB) data may be provided to the DMD 604 one color at
a time, such that each frame of data is divided into red, blue, and
green data segments. Typically, the display time for each segment
is synchronized to an optical filter, such as a color wheel 607,
which rotates so that the DMD 604 displays the data for each color
through the color wheel 607 at the proper time. Thus, the data
channels for each color are time-multiplexed so that each frame has
sequential data for the different colors.
[0058] For a sequential color system, such as the system 600
illustrated in FIG. 6, a light source 609 provides white light
through a condenser lens 610a, which focuses the light to a point
on the rotating color wheel 607. A second lens 610b may be employed
to fit the colored light output from the color wheel 607 to the
size of the pixel array on the DMD 604. Reflected light from the
DMD 604 is then transmitted to a display lens 611. The display lens
611 typically includes optical components for illuminating an image
plane, such as a display screen 608.
[0059] In an alternative embodiment, the bit-planes for different
colors could be concurrently displayed using multiple SLMs, one for
each color component. The multiple color displays may then be
combined to create the final display image. Of course, a system or
method employing the principles disclosed herein is not limited to
either embodiment.
[0060] A cross section of a DMD device is illustrated in FIG. 7
showing a mirror 702 and a gap 705. In the illustrated cross
section, individual mirrors are supported by a post 703 and rotate
about a base 704. There are alternate embodiments for DMD devices;
however, each embodiment has a similar characteristic gap between
mirror elements, as do LCD-type SLM devices. FIG. 8 illustrates why
a sufficient gap is needed between two mirror elements, as without
that gap, as shown in the figure, the adjacent mirrors can
interfere with each other. Other types of SLM devices may utilize
the gap area, for example, for routing a control signal to the
picture elements.
[0061] The individual picture element could be expanded by slightly
defocusing the image, which blends the picture elements together
but results in a lower quality image. Each individual picture
element is the lowest quantum of the image and in essence
represents a level of luminance and a color. A desired approach is
to expand the individual projected picture element to fill the gap
between them while maintaining a quality image.
[0062] FIG. 9 illustrates an embodiment in which a mirror is
attached to the shaft of a motor so that the mirror rotates within
the projection path at an angle relative to the perpendicular axis
of the motor shaft. The figure illustrates the rotating mirror
assembly 900 where a motor 905 has a mirror 902 attached to the
motor shaft. Line 904 shows the perpendicular axis to the mirror
plane, which is tilted at an angle .THETA. 903 relative to a
perpendicular line 906 to the motor shaft. As the motor shaft to
which the mirror is attached rotates, the mirror is angularly
displaced by -.THETA. to +.THETA..
[0063] FIG. 10 illustrates a basic light beam path through an
optical system 1000. The illustrations of the light path are
intended to present a simplified view of the projection system in
order to illustrate the embodiment presented here and in practical
designs; there are typically additional lenses, filters and other
components. A conventional optical path is designed so that the
light passes through the lens system in straight lines to minimize
distortions. In the system illustrated in FIG. 10, light is emitted
from the light source 1006, and the light is collimated by
converging lens 1003 so as to distribute onto the DMD device 604.
An image is formed on the DMD device and light is selectively
reflected from the DMD device onto the mirror 902. Mirror 902 is
attached to the motor 905 at an angle .THETA. relative to a
perpendicular line to the motor shaft. As the mirror 902 rotates,
the angle of the mirror changes from -.THETA.+.THETA., and given
that the law of reflection states that the angle of incidence
equals the angle of reflection, the light beam leaving the mirror
also varies from -.THETA.+.THETA.. When the optical path deviates
from a path perpendicular to the lens plane, also known as the
normal to the lens plane, the resulting image may be slightly
irregular and could become noticeable to a viewer if the angle is
too large.
[0064] FIG. 11 illustrates the effect of changing an angle anywhere
in the optical plane including the final projection plane. From
FIG. 11, it is illustrated from the principle of conic sections
that a change from a plane intersecting a right circular cone other
than perpendicular to the axis of the cone will result in the image
being stretched along one axis as in a circle transforming into an
ellipse. As illustrated in FIG. 11, a plane 1102 intersecting the
cone at a perpendicular line produces a circle, while a plane 1103
intersecting at a tilted angle would produce an ellipse or other
conic if the angle is greater. Additionally, the projection from
the rotating mirror 902 is varying from -.THETA. to +.THETA., and
the projected light traces a conic shape. At the point where the
light falls on the projection lens, the image will be rotating
about the outside perimeter of the conic that is formed. The image
that appears on the display plane 608 will also be dynamic and
rotate about the path of a conic on the display plane. Such a path
is illustrated in FIG. 12 showing variations in picture element
positions.
[0065] For a small mirror gap, the rotating mirror angle .THETA.
would typically be small and for illustrative purposes, FIG. 12
indicates a circular path and does not show any distortions. Prior
to any offsets in the optical path, the FIG. 12 illustration t0
represents an individual picture element for a DMD device 702 at a
first ("t0") instant in time, the corresponding gap between picture
elements is indicated by 1203, and 1204 indicates the center of the
picture element. When the rotating mirror is placed in the optical
path and offset by angle .THETA., the entire image along with each
picture element is now offset-correlated to the motor shaft
position. A new position, t1, is shown where the picture element
has a new center 1205 that will rotate about the point 1204
corresponding to the time t0, when no offset was present. As the
motor shaft rotates by approximately 1/8 turn, or 45 degrees, the
picture element moves to a position t1. Positions t2 and t3 are
illustrated for further increments of approximately 45-degree
rotations respectively of the motor shaft. As the motor shaft
rotates and the mirror moves from -.THETA. to +.THETA., the picture
elements will sweep an area larger than a corresponding area for a
static picture element. An obvious advantage is that the dynamic
picture elements are able to direct light into the gap area making
the gap less noticeable while maintaining image clarity.
Additionally, since a control can be achieved over the location of
a dynamic image placement, more addressable spaces can be realized.
Time slices for the SLM system may be measured in bit times, which
as discussed previously, represent the shortest time for which an
individual bit plane is available for display. Synchronizing the
picture element position with the SLM can also be used to increase
the number of locations that individual bit planes can display.
[0066] FIG. 13 illustrates how a single picture element 702 with an
un-displaced center projection 1204 can be displaced in a manner to
be perceived as four picture elements. The first displacement with
new center 1305 is at time to, followed by a second location at t1,
and a third and fourth at t2 and t3 respectively. By properly
synchronizing the bit times of the SLM with the image displacement
positions, the viewer will perceive an increase in resolution.
[0067] FIG. 14 illustrates another embodiment in which a mirror in
the optical path is modulated by a voice coil at two points, where
the voice coil motion is imparted at the two points in directions
that are generally perpendicular to the mirror plane, providing an
image displacement in two axes. As illustrated, a linear actuator
1400 is shown providing a mechanism for displacing a mirror 1402 at
one of the two points. In this example, a voice coil is used as a
single phase limited motion linear actuator and consists of a
tightly wrapped coil of wire 1403 situated near a permanent magnet
1404. The permanent magnet 1404 creates a radially oriented
magnetic field and is supported by a ferromagnetic magnet 1405 as
the inner structure which also serves to complete the magnetic
field radiating through the coil of the moving member 1406 that is
attached to the mirror 1402. When a signal is applied to the coil,
a magnetic field is generated in proportion to the signal's
current, which produces an attraction to or repulsion from the
stationary permanent magnet, creating linear motion. The signal may
vary from a negative voltage to a positive voltage in varying
degrees moving the voice coil in a linear motion closer to and
farther from the stationary magnet. A voice coil actuator can be
constructed in many different forms and this embodiment shows only
one example of the voice coil type. This embodiment can use many
different forms of voice coils or other types of linear actuators
for the purpose of displacing a component in the optical assembly.
Voice coils are an excellent choice because they are accurate with
smooth control and do not experience backlash when being
positioned. A voice coil is also a practical solution given that
they are efficient, fast, compact, low cost, have a long life and a
low overall cost.
[0068] An example of using linear actuators is illustrated in FIG.
15 where a tilting mirror assembly 1500 uses two voice coils to
tilt the mirror at two attachment points 1505, 1506. Mirror 1502 is
attached at the center by a flexible joint 1507 that allows the
mirror to rotate about the x and y axes. Linear actuators 1503 and
1504 are positioned at points 1505 and 1506 respectively along the
backside of the mirror 1502 so that the motion of the linear
actuators tilts the mirror 1502 about the flexible joint in varying
degrees. Mirror assembly 1500 may be used in the optical system
1000 in lieu of the rotating mirror assembly 900. In the linear
system, the mirror can be placed in a wider variety of positions as
compared to the rotating mirror. This flexibility allows the angle
of displacement to be adjustable, thus allowing for the degree of
gap coverage to be modified. Additionally, in a system where the
resolution is increased by image displacement, the linear
displacement described offers more control over the movement,
timing and resting locations.
[0069] In another embodiment, illustrated in FIG. 16, a mirror in
the optical path is modulated by a poled piezoelectric ceramic
element or a "piezo device" at two points that also induce motion
that is generally perpendicular to the mirror plane. A piezo device
can be used as a linear actuator in a similar manner as the voice
coils of the previous embodiment. Single piezo devices typically
provide linear movement up to about 40 microns. A piezo device 1600
is shown in the figure at a point of contraction 1600c and
elongation 1600e. When a voltage of one polarity is applied to the
piezo element, the element will lengthen and its diameter will
become smaller, thereby shaping the piezo device 1600 to its
elongation dimension 1600e. As a voltage of the opposite polarity
is applied to the piezo, the element 1600 will become shorter and
wider, thereby shaping the piezo device 1600 to its contraction
dimension 1600c. At the outer dimensions of the piezo device 1600,
linear movement occurs with the elongation and contraction.
Attaching a piezo device 1600 in a similar manner as the voice coil
of the second embodiment can provide for small mirror movement in
various directions, resulting in light being projected in the
picture element gaps and reducing the screen-door effect created by
such gaps and otherwise providing a dithering effect to the
projected image.
[0070] In another embodiment illustrated in FIG. 17, a lens in the
optical path is modulated by a linear actuator at two points, which
move in a direction that is generally perpendicular to the lens
center plane, thereby providing an image displacement in two axes.
The linear actuators 1503, 1504 could be voice coils, piezoelectric
elements or another of the diverse types of linear actuators. The
reflected image from the DMD device or other SLM device is
projected through the moveable double convex lens 1702 where the
light reaches the lens surface and refracts according to the
effective angle of incidence. As the lens is moved, the effective
angle of incidence varies, resulting in changes to the light
refraction thus changing the image position. The double convex
lens, having its characteristic curved surface, will refract light
rays more at a distance farther from the lens center. Similar
dithering effects also may result from translating the points on
the lens in directions that are not perpendicular to the center
plane of the lens 1702, since this would affect the focusing of the
image through the lens. It is also possible to modulate the index
of refraction of the lens 1702 or another optical element in order
to affect the focusing of the image projected through the lens.
These types of image displacement can also be used effectively to
increase the optical resolution of or dither the projected
images.
[0071] In another embodiment, a plane-parallel plate in the optical
path is modulated by a linear actuator at two points, which are
perpendicular to lens center plane, providing an image displacement
in two axes. FIG. 18 illustrates the plane-parallel plate 1802 used
in an optical system 1800. The linear actuators 1503, 1504 could be
voice coils, piezoelectric elements or another of the diverse types
of linear actuators. Light entering a plane-parallel plate is
refracted upon entering an optical material and as the light
propagates through the plate, it again refracts at the front and
rear boundaries. As the plate 1802 is moved, the angle of
orientation between the light ray and the plate can displace the
direction of propagation moving the image. Unlike the curved lens
of the previous embodiment, the plate is flat with light rays
passing through as parallel rays and the displacement of the
picture elements will be uniform. As with the curved lens
embodiment of FIG. 17, the displacement of the image may be
effected by modulating the optical properties of the plate 1802
rather than mechanically displacing it. Various integrated optic
devices for modulating optical signals by the changing of the
devices' indexes of refraction include various electro-optic
modulators and acousto-optic modulators known in the art. The
operation of these modulators is described, for example, in ROBERT
G. HUNSBURGER, INTEGRATED OPTICS: THEORY AND TECHNOLOGY 120-57
(Springer-Verlag 1984), which is hereby incorporated herein solely
for the purpose of describing techniques and devices for the
integrated optic modulation of light without mechanical
displacement of optical elements.
[0072] Advantages of this embodiment include the uniform
displacement of all picture elements and the ability to have
excellent control over the direction and amount of image
displacement of an image. This method may be used to smooth an
image as well as create additional picture element addressable
locations.
[0073] While this invention has been described with reference to
illustrative embodiments, this description is not intended to be
construed in a limiting sense. A few embodiments have been
described in detail herein. It is to be understood that the scope
of the invention also comprehends embodiments different from those
described, yet within the scope of the claims. Words of inclusion
are to be interpreted as nonexhaustive in considering the scope of
the invention. While this invention has been described with
reference to illustrative embodiments, this description is not
intended to be construed in a limiting sense. Various modifications
and combinations of the illustrative embodiments, as well as other
embodiments of the invention, will be apparent to persons skilled
in the art upon reference to the description. It is therefore
intended that the appended claims encompass any such modifications
or embodiments.
[0074] The section headings in this application are provided for
consistency with the parts of an application suggested under 37 CFR
1.77 or otherwise to provide organizational cues. These headings
shall not limit or characterize the invention(s) set out in any
patent claims that may issue from this application. Specifically
and by way of example, although the headings refer to a "Technical
Field," the language chosen under this heading to describe the
so-called field of the invention should not limit the claims.
Further, the "Summary and Method" is not to be considered as a
characterization of the invention(s) set forth in the claims to
this application. Multiple inventions may be set forth according to
the limitations of the multiple claims associated with this patent
specification, and the claims accordingly define the invention(s)
that are protected thereby. In all instances, the scope of the
claims shall be considered on their merits in light of the
specification but should not be constrained by the headings
included in this application.
[0075] Realizations in accordance with the present invention have
been described in the context of particular embodiments. These
embodiments are meant to be illustrative and not limiting. Many
variations, modifications, additions, and improvements are
possible. Accordingly, plural instances may be provided for
components described herein as a single instance. Boundaries
between various components, operations, and data stores are
illustrated in the context of specific configurations. Other
allocations of functionality are envisioned and will fall within
the scope of claims that follow. Finally, structures and
functionality presented as discrete components in the exemplary
configurations may be implemented as a combined structure or
component. These and other variations, modifications, additions,
and improvements may fall within the scope of the invention as
defined in the claims that follow.
* * * * *